Enforcing illegal torrents failed. I think a cryptocurrency ban would be similar. Also, the US made it illegal to hold gold between 1933 and 1974 but almost nobody turned in gold to the feds. https://en.m.wikipedia.org/wiki/Executive_Order_6102

They enforce it when you buy something real with the coins. A torrent is the thing you want, it isn't a currency to buy the thing you want.

Both are essentially electronic systems of information on a computing system, a format and a protocol of communication through the internet. In terms of technical difficulty to enforce them it would be very similar.

Does the crypto community actually stop people from spending stolen coins? No, they send off alerts but the coins still get used.

I'm pretty sure that "They" was meant to refer to the Government. The Government could most certainly pass laws (or enforce existing laws) that put people in jail for accepting stolen or "dirty" coins. (Just like the Government could pass a law or re-interpret existing laws to make making owning bitcoins illegal.)